The idea is simple: set a limit on how many immigrants can move in.
Residents like their workplace as close as possible, right? So I like to build lots of housing at specific areas, near buildings that require them (for storage/non-worker buildings, I place elsewhere).
If this is implemented, we don't have to watch the colonists move in, or get distracted and suddenly, "I have one extra colonist!"
You can turn immigration on and off in the policy settings of your government building. (Consulate, capitol, etc)